1. Field of the Invention
The present invention relates to a power supply device which converts an alternating current into a DC, and reduces a harmonic distortion of an input current so as to improve a power factor.
2. Description of the Related Art
Conventionally, a capacitor input type rectifier circuit has been used as an AC-to-DC converter circuit in various fields. The capacitor input type rectifier circuit inputs an AC voltage to a diode rectifier circuit so as to obtain a ripple voltage output, and smoothens the ripple voltage output by a capacitor so as to obtain a DC voltage. However, in the capacitor input type rectifier circuit, a conduction period of an input current becomes narrow, and then, a power factor is worsened, and further, a reactive power is much. For this reason, it is not possible to effectively use a power, and the input current contains much harmonic distortions; as a result, there is a problem of fault to equipment connected to the identical power supply device. In order to solve the above problem, conventionally, a power supply device having a circuit configuration as shown in FIG. 31A has been developed as a technique for improving a power factor to reduce a harmonic distortion.
As shown in FIG. 31B, when a rectifier circuit 103 converts an AC voltage Vin from an AC power supply 101 into a ripple output voltage, the power supply device can relieve a rush of the input current Iin by an inserted reactor 102. As a result, the conduction period becomes widened, and thereby it is possible to improve a power factor and to reduce a harmonic distortion contained in the input current Iin.
As described above, the conventional power supply device shown in FIG. 31 has been used in various apparatuses because only passive components having a simple construction are inserted so as to improve a power factor.
In recent years, a power supply device as shown in FIG. 32A has been developed, which improves a power factor using active elements. The power supply device shown in FIG. 32A will be described below. In FIG. 32A, a control circuit 109 generates and outputs a signal for turning on and off a switching element 107 at a high frequency so as to form an input current into a shape of a sine wave. A reactor 106 is a high-frequency compliant reactor for forming the input current into a shape of sine wave together with the switching element 107. A diode 108 prevents an electric change changed to a smoothing capacitor 104 from reversely flowing when the switching element 107 is in an on state.
The following is a description on an operation of the above power supply device. The control circuit 109 compares a detection current from an input current detecting circuit (not shown) with a sine wave-shaped reference waveform prepared based on a power supply voltage waveform, and then, generates and outputs a pulse signal for controlling an on/off of the switching element 107 so as to form the input current into a shape of a sine wave. The switching element 107 makes an on/off operation in accordance with the pulse signal to cause the reactor 106 to be in a short circuit or in an open circuit repeatedly so that the input current is brought close to the reference waveform. As a result, as shown in FIG. 32B, it is possible to obtain a sine wave-shaped input current substantially similar to the alternating voltage Vin of the AC power supply 101, and thus, to bring a power factor close to 1. Further, it is possible to greatly reduce a harmonic distortion contained in the input current Iin.
Moreover, there is a power supply device which remarkably simplifies a switching control so as to improve a power factor, as disclosed in Japanese Patent Laid-open Publication No. 9-266674, 10-174442, or Japanese Patent No. 2-763479.
These power supply devices will be described below with reference to FIGS. 33 and 34.
In a power supply device shown in FIG. 33A, a reactor 102 is used for a low frequency. A control circuit 110 outputs a pulse signal for turning on the switching element 107 for a predetermined time in synchronous with a zero cross point of the AC power supply 101. Whereby a current for short-circuiting the AC power supply 101 flows via the rectifier circuit 103, the reactor 102 and the switching element 107, and thereby, the input current flows from the zero cross point of the AC power supply 101. Then, when the switching element 107 becomes an off state, a current flows through the rectifier circuit 103, the reactor 102, a reverse-current blocking rectifier element 108 and the smoothing capacitor 104. As a result, it is possible to be make wide a conduction period to improve a power factor. Further, the control circuit 110 can output the pulse signal for turning on the switching element 107 after a delay of predetermined time from the zero cross point of the AC power supply 101. The delay time may be set in accordance with a magnitude of load, and thereby, it is possible to obtain an optimum power factor for each load.
A power supply device shown in FIG. 34A includes capacitors 120a and 120b for improving a power factor. A control circuit 111 outputs a pulse signal for turning on a bi-directional signal 115 for a predetermined time in the vicinity of the zero cross point of the AC power supply 101. Thus a charging current flows to the capacitor 120a or 120b via a reactor 102 and a rectifier circuit 103. A phase of the charging current is advanced, it is therefore possible to make early a rise of the input current. Then, when the bi-directional switch 115 becomes off, an input current flows through the reactor 102, the rectifier circuit 103 and a smoothing circuit 104. Consequently, a conduction period of the input current can be widened to improve a power factor.
Moreover, in the power supply device shown in FIG. 34A, the control circuit 111 can change an output voltage value by changing a pulse width of the pulse signal. More specifically, the power supply device shown in FIG. 34A is operated as a full-wave rectifier circuit when the bi-directional switch is off, and is operated as a voltage doubler rectifier circuit when the bi-directional switch is in an on state.
Therefore, by changing a pulse width of the pulse signal, the control circuit 111 can change an output voltage within a range which is more than a voltage obtained a full-wave rectification, and is lower than a voltage obtained by a voltage doubler rectification.
However, in the above power supply device shown in FIG. 31A, its improvement effect is low although the power supply device can improve a power factor with a simple construction and a sufficient power factor can not be obtained. Since a reactor value must be made large in order to obtain high power factor with the circuit construction, it causes a problem that components would be made into a large size and a loss would simultaneously increase.
Even though the above power supply device shown in FIG. 32A can form the input current into a shape of sine wave and control a power factor to approximately 1, the power supply device has the following problems. More specifically, the control becomes complicate, and a loss in the switching element 107 is increased due to high frequency switching accompanying with waveform shaping, and further, a switching noise increases. Therefore a powerful filter circuit is required for restricting the aforesaid loss. This causes a cost increase, an increase of loss in the filter circuit, and finally efficiency deterioration as a whole.
The above power supply device shown in FIG. 33A has the following problems although it can remarkably simplify a switching control. More specifically, in particular, a current waveform becomes a non-continuous state as shown in FIG. 33B in a low load, or is too advanced; for this reason, a sufficient power factor can not obtained. Further, an output timing of the pulse signal is delayed for a predetermined time from the zero cross point of the AC power supply, and thereby, an optimum power factor is obtained in a low load. However, the control becomes complicate, because both of a delay time and a pulse width must be controlled. Further, in the case where the above power supply device is applied to an air conditioner or the like, the reactor is made into a large size in inputting 200V, and then, a loss is increased in the large-sized reactor. Further, since a voltage applied to switching means becomes high, a component having a high withstand voltage is required. As a result, the component is made into a large size, and a switching loss becomes great in the switching element 107.
In the above power supply device shown in FIG. 34A, it is possible to remarkably simplify a switching control, and to obtain high power factor in a low load. Further, it is possible to increase an output voltage within a range which is more than a voltage obtained by a full wave rectification, and is lower than a voltage obtained by a voltage doubler rectification. This power supply device however has the following problems. More specifically, as shown in FIG. 34B, when an output voltage increases, a voltage boost ratio becomes higher, and the power supply device is close to the voltage double rectifier circuit. Consequently a sufficient power factor can not be obtained. It is impossible to obtain an output voltage more than a voltage obtained by the voltage doubler rectification.